Link between schizophrenia and bipolar disorder found

Schizophrenia and bipolar disorder may be two versions of the same mental illness, a major study of nine million people suggests.

Scientists found that both conditions share the same genetic causes.

Understanding the links between them could improve treatments and the development of new drugs, said the researchers.

Schizophrenia is a psychotic illness that causes delusions and hallucinations. Bipolar disorder, previously known as manic depression, causes a see-sawing of mood between extreme depression and mania.

The study involved nine million individuals from two million Swedish families and covered a 30 year period from 1973 to 2004.

Family risks of inheriting a susceptibility to either of the disorders were assessed and found to be broadly similar.

Full siblings were nine times more likely than the general population to have schizophrenia and eight times more likely to have bipolar disorder.

Maternal half-siblings were 3.6 times more likely to have schizophrenia and 4.5 times more likely to have bipolar disorder.

Schizophrenia was 64% inherited rather than being caused by environmental factors, while the heritability of bipolar disorder was 59%.

Genetic factors largely accounted for cases where people had both conditions.

Authors Dr Paul Lichtenstein and Dr Christina Hultman, from the Karolinska Institute in Stockholm, wrote in the Lancet medical journal: "We showed evidence that schizophrenia and bipolar disorder partly share a common genetic cause. These results challenge the current dichotomy between schizophrenia and bipolar disorder, and are consistent with a reappraisal of these disorders as distinct diagnostic entities.

"Within clinical practice, the underlying structure of psychosis and the knowledge of the common causes of these disorders might be beneficial for treatment options and development of psychosis medication."

(Source: Mental Health Foundation UK: Lancet: February 2009)